[flax safety checker] Use `FlaxPreTrainedModel` for saving/loading (#591)

* use FlaxPreTrainedModel for flax safety module

* fix name

* fix one more

* Apply suggestions from code review

src/diffusers/pipelines/stable_diffusion/safety_checker_flax.py

 import warnings
+from typing import Optional, Tuple
 
 import numpy as np
 
@@ -6,13 +7,9 @@ import jax
 import jax.numpy as jnp
 from flax import linen as nn
 from flax.core.frozen_dict import FrozenDict
-from flax.struct import field
-from transformers import CLIPVisionConfig
+from transformers import CLIPConfig, FlaxPreTrainedModel
 from transformers.models.clip.modeling_flax_clip import FlaxCLIPVisionModule
 
-from ...configuration_utils import ConfigMixin, flax_register_to_config
-from ...modeling_flax_utils import FlaxModelMixin
-
 
 def jax_cosine_distance(emb_1, emb_2, eps=1e-12):
     norm_emb_1 = jnp.divide(emb_1.T, jnp.clip(jnp.linalg.norm(emb_1, axis=1), a_min=eps)).T
@@ -20,34 +17,17 @@ def jax_cosine_distance(emb_1, emb_2, eps=1e-12):
     return jnp.matmul(norm_emb_1, norm_emb_2.T)
 
 
-@flax_register_to_config
-class FlaxStableDiffusionSafetyChecker(nn.Module, FlaxModelMixin, ConfigMixin):
-    projection_dim: int = 768
-    # CLIPVisionConfig fields
-    vision_config: dict = field(default_factory=dict)
+class FlaxStableDiffusionSafetyCheckerModule(nn.Module):
+    config: CLIPConfig
     dtype: jnp.dtype = jnp.float32
 
-    def init_weights(self, rng: jax.random.PRNGKey) -> FrozenDict:
-        # init input tensor
-        input_shape = (
-            1,
-            self.vision_config["image_size"],
-            self.vision_config["image_size"],
-            self.vision_config["num_channels"],
-        )
-        pixel_values = jax.random.normal(rng, input_shape)
-        params_rng, dropout_rng = jax.random.split(rng)
-        rngs = {"params": params_rng, "dropout": dropout_rng}
-        return self.init(rngs, pixel_values)["params"]
-
     def setup(self):
-        clip_vision_config = CLIPVisionConfig(**self.vision_config)
-        self.vision_model = FlaxCLIPVisionModule(clip_vision_config, dtype=self.dtype)
-        self.visual_projection = nn.Dense(self.projection_dim, use_bias=False, dtype=self.dtype)
+        self.vision_model = FlaxCLIPVisionModule(self.config.vision_config)
+        self.visual_projection = nn.Dense(self.config.projection_dim, use_bias=False, dtype=self.dtype)
 
-        self.concept_embeds = self.param("concept_embeds", jax.nn.initializers.ones, (17, self.projection_dim))
+        self.concept_embeds = self.param("concept_embeds", jax.nn.initializers.ones, (17, self.config.projection_dim))
         self.special_care_embeds = self.param(
-            "special_care_embeds", jax.nn.initializers.ones, (3, self.projection_dim)
+            "special_care_embeds", jax.nn.initializers.ones, (3, self.config.projection_dim)
         )
 
         self.concept_embeds_weights = self.param("concept_embeds_weights", jax.nn.initializers.ones, (17,))
@@ -109,3 +89,59 @@ class FlaxStableDiffusionSafetyChecker(nn.Module, FlaxModelMixin, ConfigMixin):
                 )
 
         return images, has_nsfw_concepts
+
+
+class FlaxStableDiffusionSafetyChecker(FlaxPreTrainedModel):
+    config_class = CLIPConfig
+    main_input_name = "clip_input"
+    module_class = FlaxStableDiffusionSafetyCheckerModule
+
+    def __init__(
+        self,
+        config: CLIPConfig,
+        input_shape: Optional[Tuple] = None,
+        seed: int = 0,
+        dtype: jnp.dtype = jnp.float32,
+        _do_init: bool = True,
+        **kwargs,
+    ):
+        if input_shape is None:
+            input_shape = (1, 224, 224, 3)
+        module = self.module_class(config=config, dtype=dtype, **kwargs)
+        super().__init__(config, module, input_shape=input_shape, seed=seed, dtype=dtype, _do_init=_do_init)
+
+    def init_weights(self, rng: jax.random.PRNGKey, input_shape: Tuple, params: FrozenDict = None) -> FrozenDict:
+        # init input tensor
+        clip_input = jax.random.normal(rng, input_shape)
+
+        params_rng, dropout_rng = jax.random.split(rng)
+        rngs = {"params": params_rng, "dropout": dropout_rng}
+
+        random_params = self.module.init(rngs, clip_input)["params"]
+
+        return random_params
+
+    def __call__(
+        self,
+        clip_input,
+        params: dict = None,
+    ):
+        clip_input = jnp.transpose(clip_input, (0, 2, 3, 1))
+
+        return self.module.apply(
+            {"params": params or self.params},
+            jnp.array(clip_input, dtype=jnp.float32),
+            rngs={},
+        )
+
+    def filtered_with_scores(self, special_cos_dist, cos_dist, images, params: dict = None):
+        def _filtered_with_scores(module, special_cos_dist, cos_dist, images):
+            return module.filtered_with_scores(special_cos_dist, cos_dist, images)
+
+        return self.module.apply(
+            {"params": params or self.params},
+            special_cos_dist,
+            cos_dist,
+            images,
+            method=_filtered_with_scores,
+        )